Exhaust gas purifying apparatus for internal combustion engine

ABSTRACT

An exhaust gas purifying apparatus for an internal combustion engine is provided for switching a switching valve at an optimal timing in accordance with an actual activated state of a catalyzer and a heated state of an adsorbent to achieve an optimal exhaust gas characteristic. The exhaust gas purifying apparatus comprises a catalyzer disposed in an exhaust system of the internal combustion engine, an adsorbent filled in a second passage circumventing a first passage in the exhaust system for adsorbing hydrocarbons within exhaust gases, a switching valve operable to switch between an open position for opening the first passage and a close position for closing the first passage, an ECU for detecting an atmospheric pressure state, and a switching valve driver for driving the switching valve to the close position upon start of the internal combustion engine, and driving the switching valve to the open position in accordance with the detected atmospheric pressure state.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust gas purifying apparatuswhich purifies exhaust gases exhausted from an internal combustionengine, and temporarily adsorbs hydrocarbons within exhaust gases uponstart of the internal combustion engine.

2. Description of the Prior Art

The type of conventional exhaust gas purifying apparatus for an internalcombustion engine mentioned above is known, for example, from Laid-openJapanese Patent Application No. 2000-310113. The exhaust gas purifyingapparatus disclosed therein comprises a pair of upstream and downstreamthree-way catalysts in an exhaust pipe of an internal combustion engine.An inner main exhaust passage and an annular bypass passage around themain exhaust passage are formed between the two three-way catalystswithin the exhaust pipe. The bypass passage has a passage area smallerthan the main exhaust passage, and is filled with a hydrocarbonadsorbent. A switching valve is also provided at the inlet port of themain exhaust passage for opening and closing the main exhaust passage.

For controlling the switching valve, it is determined whether or not thefollowing three conditions are met after the engine is started:

1) whether or not a cooling water temperature of the engine detected bya water temperature sensor is lower than a predetermined temperature;

2) whether or not the amount of intake air detected by an air flow meteris smaller than a predetermined amount; and

3) whether or not a time elapsed after the start is shorter than acatalyst activation time which is determined in accordance with thecooling water temperature.

When the three conditions are all met, the switching valve is fullyclosed on the assumption that the downstream three-way catalyst has notbeen activated. In this state, exhaust gases passing through theupstream three-way catalyst are entirely passed to the bypass passage,so that hydrocarbons within the exhaust gases are adsorbed by theadsorbent filled in the bypass passage. Then, the exhaust gases flowinto the downstream three-way catalyst, thereby preventing hydrocarbonsfrom being emitted to the atmosphere. On the other hand, when any of thethree conditions is not met, the switching valve is fully opened on theassumption that the downstream three-way catalyst has been activated. Inthis state, a madownstream three-way catalyst for purification throughits oxidation/reduction actions.

However, the conventional exhaust gas purifying apparatus determineswhether the downstream three-way catalyst is activated after the startof the engine based on the cooling water temperature, amount of absorbedair, and time elapsed after the start, which are used as parameters, sothat the exhaust gas purifying apparatus may fail to make appropriatedetermination, resulting in the inability to switch the switching valveat an appropriate timing. For example, since the downstream three-waycatalyst is located substantially away from the engine body for whichthe cooling water temperature is detected, the temperature of thedownstream three-way catalyst rises with a delay from the cooling watertemperature. As such, the cooling water temperature does not alwaysmatch the temperature of the downstream three-way catalyst in risingtiming, behavior and the like. Therefore, the cooling water temperaturemay not exactly reflect the actual temperature state of the downstreamthree-way catalyst, i.e., whether it is activated.

To solve the disadvantage as mentioned above, the temperature of thedownstream three-way catalyst may be directly detected by a temperaturesensor for use as a parameter instead of the cooling water temperature.With this strategy, however, the activation of the three-way catalystcannot either detected with high accuracy because the temperature sensorgenerally has a responsibility too low for use with the downstreamthree-way catalyst which is activated in a relatively short time afterthe start of the engine, and also because the temperature sensorexperiences difficulties in detecting the temperature at the center ofthe downstream three-way catalyst, which is critical for evaluatingwhether or not the three-way catalyst is activated, in a temperaturedistribution of the three-way catalyst which can readily vary when thetemperature rises in such a short time.

Also, since the temperature rising rate of the downstream three-waycatalyst depends on a particular operating condition after the start ofthe engine (for example, when the vehicle is idled after the start, andwhen the vehicle is launched immediately after the start), the timeelapsed after the start does not either reflect exactly an actualactivated state of the downstream three-way catalyst. Further, in regardto the amount of intake air, since a detection value detected everypredetermined time is compared with a predetermined value, theconventional exhaust gas purifying apparatus will erroneously determinethat the downstream three-way catalyst has been activated if the amountof intake air instantaneously increases. From the result of theforegoing analysis, the conventional exhaust gas purifying apparatuscannot set a timing at which the switching valve is switched to the mainexhaust passage appropriately in response to a transition of thedownstream three-way catalyst into activation. Consequently, if theswitching valve is switched at a timing too early, exhaust gases willflow into the inactivated downstream three-way catalyst, so thathydrocarbons will be emitted to the atmosphere to exacerbate the exhaustgas characteristic. On the other hand, if the switching valve isswitched at a timing too late, exhaust gases will flow into thedownstream three-way catalyst with a delay, though it has been alreadyactivated, thereby failing to effectively utilize the purifyingperformance.

Also, a substantial amount of intake air reduced in a highlandenvironment, as compared with a flatland environment, results in a lowercombustion temperature which fails to sufficiently raise the temperatureof exhaust gases, causing a delay in the activation of the three-waycatalyst and a lower rate at which the adsorbent is heated. For thisreason, a longer time is required for the adsorbent to be heated to atemperature at which hydrocarbons can be desorbed therefrom in thehighland environment than in the flatland environment. The prior art,however, does not take into account the temperature rise characteristicsof the three-way catalyst or adsorbent in such a highland environment,so that exhaust gases flowing into the inactivated downstream three-waycatalyst will exacerbate the exhaust gas characteristics. In addition,since the switching valve is switched to the main exhaust passage beforethe desorption temperature is reached, the desorption actually startswith a delay, resulting in a longer time required to completely desorbhydrocarbons from the adsorbent. With such a delay in completing thedesorption, if the engine is stopped immediately after it has beenstarted, for example, hydrocarbons, which are not desorbed, remainwithin the adsorbent, so that the adsorbent fails to sufficientlydemonstrate the adsorption performance at the time the engine is nextstarted.

OBJECT AND SUMMARY OF THE INVENTION

The present invention has been made to solve the problems as mentionedabove, and it is an object of the invention to provide an exhaust gaspurifying apparatus for an internal combustion engine which is capableof switching a switching valve at an optimal timing in accordance withan actually activated state of a catalyzer to achieve an optimal exhaustgas characteristic.

To achieve the above object, the present invention provides an exhaustgas purifying apparatus for an internal combustion engine for purifyingexhaust gases discharged from the internal combustion engine, andtemporarily adsorbing hydrocarbons within exhaust gases upon start ofthe internal combustion engine. The exhaust gas purifying apparatus ischaracterized by comprising a catalyzer disposed in an exhaust system ofthe internal combustion engine for purifying exhaust gases, an adsorbentfilled in a second passage in the exhaust system for adsorbinghydrocarbons within exhaust gases, the second passage circumventing afirst passage, a switching valve operable to switch between an openposition for opening the first passage and a close position for closingthe first passage, atmospheric pressure state detecting means fordetecting an atmospheric pressure state, and switching valve drivingmeans for driving the switching valve to the close position upon startof the internal combustion engine, and for driving the switching valveto the open position in accordance with the detected atmosphericpressure state.

According to this exhaust gas purifying apparatus for an internalcombustion engine, the switching valve is driven to the close positionto close the first passage upon start of the internal combustion engine,thereby forcing exhaust gases to flow into the second passage whichcircumvents the first passage. In this way, hydrocarbons within theexhaust gases are adsorbed by the adsorbent filled in the second passageto prevent the emission of the hydrocarbons to the atmosphere.

Subsequently, the switching valve is driven to the open position to openthe first passage in accordance with a detected atmospheric pressurestate. Consequently, exhaust gases are guided to flow into the firstpassage, and the adsorbent finishes adsorbing hydrocarbons. The exhaustgases are purified by the activated catalyzer before they are emitted tothe atmosphere. Also, since the switching valve is driven to the openposition in accordance with the atmospheric pressure state, theswitching valve can be switched at a timing which is set later thanusual as the atmospheric pressure is lower, for example, on a highland,to increase the calory applied to the adsorbent and catalyzer during theadsorption of hydrocarbons, thereby sufficiently heating the adsorbentand catalyzer. As a result, even on a highland, it is possible tomaximally activate the catalyzer, hasten the desorption of hydrocarbonsfrom the adsorbent, and permit sufficient desorption of hydrocarbons ina short time.

Preferably, the exhaust gas purifying apparatus for an internalcombustion engine further comprises start-time temperature statedetecting means for detecting a temperature state of the exhaust systemupon start of the internal combustion engine, and post-start exhaust gascalory calculating means for calculating the calory of exhaust gasesdischarged after the start of the internal combustion engine, whereinthe switching valve driving means drives the switching valve to the openposition in accordance further with the detected start-time temperaturestate of the exhaust system, and the calculated post-start exhaust gascalory.

The start-time temperature state is a parameter indicative of atemperature state of the exhaust system and the catalyzer providedtherein upon start of the internal combustion engine, while thepost-start exhaust gas calory exactly reflects the temperature state,i.e., activated state of the catalyzer after the start. Therefore,according to this preferred embodiment of the exhaust gas purifyingapparatus, since the switching valve is driven to the open position inaccordance further with the detected start-time temperature state of theexhaust system, and the calculated post-start exhaust gas calory, theactivated state of the catalyzer can be evaluated based on the parameterindicative of the temperature state only upon starting, and evaluatedbased on the exhaust gas calory used as a parameter after the startbased on the temperature state. It is therefore possible to highlyaccurately determine the activated state of the catalyzer while avoidingthe inaccuracy which would be resulted when the activated state isdetermined using the result of detection by a temperature sensor afterthe start. Consequently, the switching valve can be driven to the openposition at an optimal timing immediately after the catalyzer isactually activated, thereby achieving an optimal exhaust gascharacteristic.

Preferably, in the exhaust gas purifying apparatus for an internalcombustion engine, the start-time temperature state detecting meansincludes stop-time temperature detecting means for detecting thetemperature of the exhaust system at the preceding stop of the internalcombustion engine, and inoperative time measuring means for measuring aninoperative time from the preceding stop to the current start of theinternal combustion engine, wherein the start-time temperature statedetecting means is configured to find the start-time temperature stateof the exhaust system in accordance with the detected stop-timetemperature of the exhaust system and the measured inoperative time.

The temperature in the exhaust system upon stop of the internalcombustion engine depends on an operating condition including anoperating time of the internal combustion engine until the stop, and thetemperature in the exhaust system after the stop varies from thisstop-time temperature in accordance with a time elapsed from the stop.Thus, according to this preferred embodiment of the exhaust gaspurifying apparatus, the start-time temperature state in the exhaustsystem can be highly accurately detected in accordance with thepreceding operating condition of the internal combustion engine, and theduration in which the engine has been inoperative. Thus, the activatedstate of the catalyzer can be determined with higher accuracy inaccordance with the start-time temperature state of the exhaust system,so that the switching valve can be more appropriately switched inaccordance with the activated state of the catalyzer.

Preferably, in the exhaust gas purifying apparatus for an internalcombustion engine, the start-time temperature state detecting meansfurther includes an ambient temperature detecting means for detectingthe ambient temperature around the internal combustion engine, whereinthe start-time temperature state detecting means is configured to findthe start-time temperature state of the exhaust system in accordancefurther with the detected ambient temperature.

The temperature in the exhaust system after the internal combustionengine is stopped varies depending not only on the length of elapsedtime after the stop but also on the ambient temperature around theinternal combustion engine, and varies at a larger rate, for example,when there is a larger difference between the temperature in the exhaustsystem upon stop of the internal combustion engine and the ambienttemperature. Thus, according to this preferred embodiment of the exhaustgas purifying apparatus, the ambient temperature around the internalcombustion engine is employed as an additional parameter to moreaccurately detect the start-time temperature state of the exhaustsystem, thereby making it possible to more appropriately determinewhether the catalyst is activated and switch the switching valve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram generally illustrating the configuration of aninternal combustion engine in which an exhaust gas purifying apparatusis applied according to a first embodiment of the present invention;

FIG. 2 is an enlarged cross-sectional view illustrating a hydrocarbonadsorbing device;

FIG. 3 is a flow chart illustrating a routine for making settings uponstart of the engine;

FIG. 4 shows a cooling coefficient table for setting a coolingcoefficient KCOOL in accordance with a soak time TMSOAK;

FIG. 5 is a graph showing an exemplary change in a start catalysttemperature TCAT_INI in accordance with a stop catalyst temperatureTCAT_LAST, the soak time TMSOAK, and an external air temperature TA;

FIG. 6 is an exhaust gas calory initial value table for setting aninitial value SGM_Q_INI for an accumulated exhaust gas calory valueSGM_Q in accordance with the start catalyst temperature TCAT_INI;

FIG. 7 is a graph showing the relationship among the accumulated exhaustgas calory value SGM_Q, its initial value SGM_Q_INI, and a determinationvalue TMTRSTIM;

FIG. 8 is a flow chart illustrating a routine for controlling theswitching valve to open and close;

FIG. 9 is a flow chart illustrating a routine for calculating theaccumulated exhaust gas calory value SGM_Q;

FIG. 10 is a determination value table for setting the determinationvalue TMTRSTIM in accordance with the atmospheric condition; and

FIG. 11 is block diagram illustrating the configuration of an exhaustgas purifying apparatus for an internal combustion engine according to asecond embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, a preferred embodiment of the present invention willbe described in detail with reference to the accompanying drawings. FIG.1 illustrates an internal combustion engine (hereinafter called the“engine”) in which an exhaust gas purifying apparatus is appliedaccording to a first embodiment of the present invention. The engine 1is, for example, a four-cylinder four-cycle engine equipped in avehicle, not shown. The engine 1 comprises an exhaust system 2 which hasan exhaust pipe 4 connected to the engine 1 through an exhaust manifold3. A catalyzer 6 having two three-way catalysts 5, and a hydrocarbonadsorber 7 for adsorbing hydrocarbons are provided halfway in theexhaust pipe 4. The two three-way catalysts 5 of the catalyzer 6 arearranged adjacent to each other along the exhaust pipe 4, and purifyharmful substances (hydrocarbons (HC), carbon monoxide (CO) and nitrogenoxides (NOx)) within exhaust gases passing through the catalyzer 6 byoxidation-reduction catalyst actions, when they are heated to apredetermined temperature (for example, 300 C) or higher and activated.

The hydrocarbon adsorber 7 in turn is arranged at a location downstreamof the catalyzer 6 in the exhaust pipe 4, and provided for reducing theamount of hydrocarbons emitted to the atmosphere by adsorbinghydrocarbons within exhaust gases during a starting period (for example,for approximately 30 to 40 seconds from the start) of the engine 1 in acold state in which the three-way catalysts 5 have not been activated.As illustrated in FIGS. 1 and 2, the hydrocarbon adsorber 7 is coupledto a downstream end of the catalyzer 6 through an exhaust passage switch8. The hydrocarbon adsorber 7 comprises a cylindrical case 11; a bypassexhaust pipe 12 arranged within the case 11; and a cylindrical adsorbent16 arranged halfway in the bypass exhaust pipe 12 for adsorbinghydrocarbons in exhaust gases which are introduced into the bypassexhaust pipe 12.

As illustrated in FIG. 2, a main passage 13 (first passage) is definedby an annular space in cross section between the case 11 and bypassexhaust pipe 12, while a bypass passage 14 (second passage) is definedby an internal space of the bypass exhaust pipe 12. The case 11 has itsupstream end-divided into two, i.e., an upper and a lower opening 11 a,11 b. The upper opening 11 a is in communication with the main passage13, while the lower opening 11 b is in communication with the bypasspassage 14.

The bypass exhaust pipe 12 has its upstream end connected to an innersurface of the lower opening 11 b of the case 11, and a downstream endconnected to an inner surface of a downstream end of the case 11,respectively, in an air tight state. The bypass exhaust pipe 12 isformed with a plurality (for example, five) of elongated communicationholes 12 a in a downstream end portion in the circumferential directionat equal intervals, such that the downstream end of the main passage 13is in communication with the downstream end of the bypass passage 14through these communication holes 12 a.

The adsorbent 16 is comprised of a honeycomb core (not shown), made of ametal, which carries zeolite on its surface, and has the property ofadsorbing moisture as well as hydrocarbons. As exhaust gases introducedinto the bypass passage 14 pass through the adsorbent 16, hydrocarbonsand moisture in the exhaust gases are adsorbed by the zeolite. Thezeolite, which has high heat resistant properties, adsorbs hydrocarbonsat low temperatures (for example, below 100 C), and desorbs hydrocarbonsonce adsorbed thereby at a predetermined temperature or higher (forexample, 100-250 C).

The exhaust passage switch 8 is provided for selectively switching thepassage of exhaust gasses downstream of the catalyzer 6 to the mainpassage 13 or to bypass passage 14 in accordance with an activated stateof the three-way catalysts 5. The exhaust passage switch 8 comprises acylindrical coupling pipe 18; and a pivotable switching valve 15arranged in the coupling pipe 18. The switching valve 15 is driven by aswitching valve driver 19 (see FIG. 1) which is controlled by an ECU 25,later described, for switching the exhaust gas passage to the mainpassage 13 when it is present at a position indicated by solid lines inFIG. 2 (open position) and for switching the exhaust gas passage to thebypass passage 14 when it is present at a position indicated by two-dotchain lines (close position).

The EGR pipe 17 is also coupled between the coupling pipe 18 and theintake pipe 1 a of the engine 1 for recirculating a portion of exhaustgases to the engine 1, and an EGR control valve 20 is arranged halfwayin the EGR pipe 17. The EGR control valve 20 is controlled by the ECU 25to actuate and stop the EGR and control an EGR amount.

In the foregoing configuration, the exhaust gas passage is switched tothe bypass passage 14 by the exhaust passage switch 8 immediately aftera cold start of the engine 1, thereby leading exhaust gasses passingthrough the catalyzer 6 to the bypass passage 14. The exhaust gases areemitted to the atmosphere after hydrocarbons in the exhaust gases havebeen adsorbed by the adsorbent 16. Subsequently, as hydrocarbons havebeen adsorbed by adsorbent 16, the exhaust gas passage is switched tothe main passage 13 by the switching valve 15 to lead the exhaust gasesto the main passage 13 through the coupling pipe 18 for emission to theatmosphere. Also, as the EGR control valve 20 is opened to operate theEGR, a portion of the exhaust gases is recirculated to the intake pipe 1a through the bypass passage 14 and EGR pipe 17 as an EGR gas.Hydrocarbons desorbed from the adsorbent 16 are sent to the intake pipe1 a by the EGR gas and burnt by the engine 1.

A humidity sensor 22 is attached to the case 11 of the hydrocarbonadsorber 7 at a location downstream of the adsorbent 16. The humiditysensor 22, which is integrated with a temperature sensor, includes asensor element 22 a which faces the bypass passage 14 for detecting ahumidity VRST and a temperature THCM within the bypass passage 14 at thelocation at which it is mounted, and outputting detection signals to theECU 25.

An engine water temperature sensor 23 and a crank angle sensor 24 arealso attached to the body of the engine 1. The engine water temperaturesensor 23 detects an engine water temperature TW, which is thetemperature of cooling water circulating within a cylinder block of theengine 1 (hereinafter called the “engine water temperature”), and sendsa detection signal indicative of the engine water temperature TW to theECU 25. The crank angle sensor 24, on the other hand, outputs a CRKsignal and a TDC signal, which are pulse signals, to the ECU 25 everypredetermined crank angle as a crack shaft, not shown, of the engine 1is rotated. An intake pressure sensor 26 is attached to the intake pipe1 a for detecting an absolute pressure PB within the intake pipe 1 a andsending a detection signal indicative of the absolute pressure PB to theECU 25. An atmospheric pressure sensor 30 is provided for detecting anatmospheric pressure PA and outputting a detection signal indicative ofthe atmospheric pressure PA to the ECU 25. An external air temperaturesensor 27 (ambient temperature detecting means) is also provided fordetecting an external air temperature TA and outputting a detectionsignal indicative of the external air temperature TA to the ECU 25. Analarm lamp 28 is further connected to the ECU 25 for generating an alarmby lighting when it is determined that the absorbent 16 is deteriorated.

In this embodiment, the ECU 25 functions as a switching valve drivingmeans, a starting temperature state detecting means, a post-startingexhaust gas calory calculating means, and a stop temperature detectingmeans. The ECU 25 is based on a microcomputer which comprises an I/Ointerface, a CPU, a RAM including a backup RAM, a ROM and the like(neither not shown). The ECU 25 further comprises a post-stop timer 25 afor measuring a time after the engine 1 is stopped, and the like. Thedetection signals from a variety of the aforementioned sensors areinputted to the CPU after they are A/D converted and reshaped in the I/Ointerface.

The CPU determines an operating condition of the engine 1 based on theengine parameter signals detected by a variety of sensors, calculates afuel injection time Tout in accordance with the result of thedetermination in synchronism with the generation of the TDC signal, andoutputs a driving signal based on the result of the calculation to aninjector 1 b. The CPU also executes various routines such as one forcontrolling the switching valve driver 19 to open and close theswitching valve 15.

Next, the routine for controlling opening/closing of the switching valve15 will be described with reference to FIGS. 3 to 10. FIG. 3 illustratesa starting process which is executed only once when the engine 1 isstarted.

In the starting process, the CPU determines at step 1 (labeled as “S1”in the figure. The same designation is applied to the followingdescription) and step 2 whether or not the engine water temperature TWis higher than a predetermined lower limit value #TWTRSL (for example,−20 C) and is lower than a predetermined upper limit value #TWTRSH (forexample, 50 C), respectively. If any of the answers to steps 2, 3 is NO,indicating that TW≦#TWTRSL or TW≧#TWTRSH, the CPU set an executionenable flag F_TRSRUN to “0” on the assumption that the engine 1 fallsout of a temperature range suitable for controlling the adsorption ofhydrocarbons by the adsorbent 16 so that the adsorption controlexecution condition is not met, followed by termination of the startingprocess. Consequently, the execution of the adsorption control isdisabled.

On the other hand, if the answers at both steps 1, 2 are YES, indicatingthat #TWTRSL<TW<#TWTRSH, the CPU sets the execution enable flag F_TRSRUNto “1” on the assumption that the adsorption control execution conditionis met (step 4).

Next, the CPU sets the value of a post-stop timer 25 a at that time asan inoperative time (hereinafter called the “soak time”) TMSOAK from theprevious stop to the current start of the engine 1 (step 5). Next, theCPU sets a cooling coefficient KCOOL by searching a cooling coefficienttable shown in FIG. 4 in accordance with the soak time TMSOAK (step 6).The cooling coefficient KCOOL represents a theoretical value for anaging change in the ratio of a change value in temperature difference toits initial value. The cooling coefficient table lists such values inthe form of a table. Therefore, the cooling coefficient KCOOL is set to1.0 when the soak time TMSOAK is zero, set to 0 when the soak timeTMSOAK is equal to or longer than a predetermined time #TMS1 (completesoak), and set to gradually decrease from 1.0 to 0 when the soak timeTMSOAK does not reach the predetermined time #TMS1 (intermediate soak).

Next, the CPU uses the cooling coefficient KCOOL set at step 6 tocalculate the temperature of the three-way catalysts 5 in the catalyzer6 upon start of the engine 1 (hereinafter called the “start-timecatalyst temperature”) TCAT_INI (step 7) in accordance with thefollowing equation (1):TCAT _(—) INI=(TCAT_LAST−TA)*KCOOL+TA  (1)where TA is the external air temperature detected by the external airtemperature sensor 27, and TCAT_LAST is the temperature of the three-waycatalysts 5 when the engine 1 was stopped the last time (hereinaftercalled the “stop-time catalyst temperature”). The stop-time catalysttemperature TCAT_LAST is estimated in a manner described below duringthe previous operation of the engine 1, and is stored in the backup RAMof the ECU 25 upon stop of the engine 1.

FIG. 5 shows an exemplary change in the start-time catalyst temperatureTCAT_INI calculated in the foregoing manner. Specifically, assuming thatthe temperature TCAT of the three-way catalysts 5 (hereinafter calledthe “catalyst temperature”) rises associated with the operation of theengine 1 and reaches the stop-time catalyst temperature TCAT_LAST uponstop of the engine 1, the catalyst temperature TCAT gradually decreasesfrom the stop-time catalyst temperature TCAT_LAST to the external airtemperature TA which is the ambient temperature therearound, and finallyconverges to the external temperature TA. In this event, the catalysttemperature TCAT follows a change in the cooling coefficient KCOOL inaccordance with the soak time TMSOAK at a changing rate proportional tothe temperature difference between the stop-time catalyst temperatureTCAT_LAST and external air temperature TA. Therefore, with theaforementioned equation (1), the start-time catalyst temperature TCAT_INcan be appropriately calculated in accordance with the stop-timecatalyst temperature TCAT_LAST, external temperature TA and soak timeTMSOAK not only for complete soak but also for intermediate soak.

Turning back to FIG. 3, at step 8 subsequent to step 7, the CPU sets aninitial value SGM_Q_INI for the accumulated exhaust gas calory valueSGM_Q by searching an exhaust gas calory initial value table shown inFIG. 6 in accordance with the start-time catalyst temperature TCAT_INIcalculated as described above, followed by termination of the startingprocess. In the exhaust gas calory initial value table, the initialvalue SGM_Q_INI is set at a smaller value as the start-time catalysttemperature TCAT_INI is lower. As described below, the accumulatedexhaust gas calory value SGM_Q (post-start exhaust gas calory) is usedas a parameter indicative of an activated state of the three-waycatalyst 5. The three-way catalyst 5 is determined to be activated whenthe accumulated exhaust gas calory value SGM_Q exceeds a redetermineddetermination value #TMTRSTIM corresponding to the temperature at whichthe three-way catalyst 5 is activated. Also, as shown in FIG. 7, thecalory of exhaust gases required to bring the catalyst temperature TCATto the activation temperature varies depending on the start-timecatalyst temperature TCAT_INI, and is larger as the TCAT_INI value islower. It is therefore possible to appropriately determine theactivation of the three-way catalyst 5 by setting the initial valueSGM_Q_IN for the accumulated exhaust gas calory value SGM_Q inaccordance with the start-time catalyst temperature TCAT_INI in theforegoing manner.

Next, at step 9 subsequent to step 8, the CPU searches a determinationvalue table shown in FIG. 10 to set the determination value TMTRSTIM inaccordance with the atmospheric pressure PA, followed by termination ofthe starting process. In this table, the determination value TMTRSTIM isset to a larger value as the atmospheric pressure PA is lower.Specifically, the determination value TMTRSTIM is set to a firstpredetermined value TMTRSTIM1 when the atmospheric pressure PA is 760mmHg (on flatland) or higher; set to a second predetermined valueTMTRSTIM2 larger than the first predetermined value TMTRSTIM1 when theatmospheric pressure PA is 500 mmHg or lower; and set to linearlydecrease from the second predetermined value TMTRSTIM2 to the firstpredetermined value TMTRSTIM1 when the atmospheric pressure PA isbetween 500 mmHg and 760 mmHg.

FIG. 8 illustrates a routine for controlling opening/closing of theswitching valve 15. This routine is executed every predetermined time(for example, every 100 ms) after the start of the engine 1. In theswitching valve control routine, the CPU first estimates the catalysttemperature TCAT and sets this value as the stop-time catalysttemperature TCAT_LAST at step 11. The stop-time catalyst temperatureTCAT_LAST thus set and updated every predetermined time is stored in thebackup RAM of the ECU 25, and is applied to the aforementioned equation(1) at step 7 in FIG. 3 the next time the engine 1 is started. Theestimate of the catalyst temperature TCAT is derived bythermodynamically modelling the exhaust system 2, determining thetemperature at an exhaust outlet port of the engine 1, i.e., theupstream end temperature of the exhaust system 2, based on an absoluteintake pipe internal temperature PB and engine rotational speed NE, andsequentially calculating the downstream temperature of the modeledexhaust system 2 based on the upstream end temperature. By thuscalculating the catalyst temperature TCAT, the temperature sensor fordetecting the catalyst temperature TCAT can be removed.

Next, the routine proceeds to step 12, where the CPU calculates theaccumulated exhaust gas calory value SGM_Q. This accumulated exhaust gascalory value SGM_Q represents the accumulated calory of exhaust gaseswhich are exhausted after the engine 1 is started. FIG. 9 illustrates asubroutine for calculating the accumulated exhaust gas calory valueSGM_Q.

In the SGM_Q calculating subroutine, the CPU first reads a fuelinjection time Tout of the injector lb for each cylinder (step 21).Then, the CPU adds the read fuel injection time Tout to the precedingversion of the accumulated exhaust gas calory value SGM_Q, and sets theresulting sum as the current accumulated exhaust gas calory value SGM_Q(step 22). In this event, immediately after the engine 1 is started, theinitial value SGM_Q_INI calculated at step 8 in FIG. 3 is used for thepreceding version of the accumulated exhaust gas calory value SGM_Q.

Turning back to FIG. 8, at step 13 subsequent to step 12, the CPUdetermines whether or not the execution enable flag F_TRSRUN is “1.” Ifthe answer to step 13 is NO, indicating that the adsorption controlexecution condition is not met, the CPU sets a switching valve flagF_TRSSOL to “0” (step 14), followed by termination of the switchingvalve control routine. By setting the switching valve flag F_TRSSOL to“0,” the switching valve driver 19 drives the switching valve 15 to theopen position to open the main passage 13 and close the bypass passage14. In this way, exhaust gases are introduced only into the main passage13, thereby disabling the adsorbent 16 to adsorb hydrocarbons.

If the answer at step 13 is YES, indicating that the adsorption controlexecution condition is met, the CPU determines whether or not theaccumulated exhaust gas calory value SGM_Q calculated at step 12 islarger than a predetermined determination value #TMTRSTIM (step 15). Ifthe answer to step 15 is NO, indicating that the accumulated exhaust gascalory value SGM_Q has not reached the determination value #TMTRSTIM,the switching valve control routine proceeds to step 16 on theassumption that sufficient calory of exhaust gases has not been providedto the three-way catalyst 5 so that the three-way catalyst 5 has notbeen activated. At step 16, the CPU determines whether or not apost-start time TMACR measured by a post-start timer (not shown) islarger than a predetermined limit time #TMTRSLMT (for example, 90seconds). Then, if the answer to step 16 is NO, the CPU sets theswitching valve flag F_TRSSOL to “1” (step 17), followed by terminationof the switching valve control routine. By setting F_TRSSOL to “1,” theswitching valve 15 is switched to the close position to close the mainpassage 13 and open the bypass passage 14. In this way, exhaust gasesare passed to the adsorbent 16 which adsorbs hydrocarbons within theexhaust gases.

On the other hand, if the answer to step 15 is YES, indicating that theaccumulated exhaust gas calory value SGM_Q exceeds the determinationvalue #TMTRSTIM, the CPU executes step 14 on the assumption that thethree-way catalyst 5 has been activated by sufficient calory of exhaustgases supplied to the three-way catalyst 5, and that the adsorbent 16has been heated to a temperature at which hydrocarbons can be desorbedtherefrom. This concludes the operation of the adsorbent 16 foradsorbing hydrocarbons, followed by the start of the operation fordesorbing adsorbed hydrocarbons. Then, desorbed hydrocarbons are sent tothe intake pipe 1 a through the EGR pipe 17 together with an EGR gas,and burnt by the engine 1.

If the answer to step 16 is YES, i.e., when the limit time #TMTRSLTMelapses before the accumulated exhaust gas calory value SGM_Q reachesthe determination value #TMTRSTIM after the start of the engine 1, theCPU executes step 14 on the assumption that the adsorbent 16 shouldterminate the adsorption of hydrocarbons, followed by termination of theswitching valve control routine.

As described above in detail, according to the foregoing embodiment, theCPU calculates the start-time catalyst temperature TCAT_INI indicativeof a temperature state of the three-way catalyst 5 upon start of theengine 1, calculates the initial value SGM_Q_INI for the accumulatedexhaust gas calory value SGM_Q in accordance with the calculatedstart-time catalyst temperature TCAT_INI, and adds the calory (Tout) ofexhaust gases applied to the three-way catalyst 5 after the start of theengine 1 to the initial value SGM_Q_INI to calculate the accumulatedexhaust gas calory value SGM_Q. Consequently, the accumulated exhaustgas calory value SGM_Q exactly reflects the temperature state, i.e.,activated state of the three-way catalyst 5.

Then, the CPU determines that the three-way catalyst 5 is activated whenthe accumulated exhaust gas calory value SGM_Q exceeds the predetermineddetermination value #TMTRSTIM indicative of the temperature at which thethree-way catalyst 5 is activated, and switches the switching valve 15from the bypass passage 14 to the main passage 13. In the foregoingmanner, the activated state of the three-way catalyst 5 is evaluated bythe start-time catalyst temperature TCAT_INI only upon starting, and isevaluated by the accumulated exhaust calory value SGM_Q_INI, based onthe start-time catalyst temperature TCAT_INI, after the starting,thereby making it possible to accurately determine the activated stateof the three-way catalyst while avoiding inaccuracy which would resultfrom a determination that is made using a detection result of thetemperature sensor after the start of the engine 1. Consequently, theswitching valve 15 can be switched to the main passage 13 in accordancewith an actual activated state of the three-way catalyst 5 at an optimaltiming immediately after it is activated, thereby achieving an optimalexhaust gas characteristic.

Also, since the start-time catalyst temperature TCAT_INI is calculatedin accordance with the stop-time catalyst temperature TCAT_LAT detectedat the last time the engine was stopped, soak time TMSOAK, and externalair temperature TA which is the ambient temperature, the start-timecatalyst temperature TCAT_INI can be found with high accuracy inaccordance with the previous operating condition of the engine 1, andthe soak time SOAK. It is therefore possible to further improve theaccuracy of determining the activated state of the three-way catalyst 5,based on the start-time catalyst temperature TCAT_INI, and to furtherappropriately set the switching timing for the switching valve 15.

Further, as described above, since the determination value TMTRSTIM forthe accumulated exhaust calory value SGM_Q is set to a larger value asthe atmospheric pressure PA is lower, the switching valve 15 is switchedat a later timing on the highland than on the flatland, thereby applyingmore calory to the adsorbent 16 and three-way catalyst 5 during theadsorption of hydrocarbons to sufficiently heat the adsorbent 16 andthree-way catalyst 5. This promotes the activation of the three-waycatalyst 5, hastens the desorption of hydrocarbons from the adsorbent16, and permits sufficient desorption of hydrocarbons in a short time.From the foregoing, the switching valve 15 can be switched from thebypass passage 14 to the main passage 13 at an optimal timing inaccordance with how the three-way catalyst is actually activated and towhat temperature the adsorbent 16 is heated, thereby achieving anoptimal exhaust gas characteristic.

In the foregoing embodiment, while the determination value TMTRSTIM isset in accordance with the atmospheric pressure PA, the accumulatedexhaust calory value SGM_Q may be corrected to be smaller as theatmospheric pressure PA is lower. Further, in the foregoing embodiment,the start-time catalyst temperature TCAT_INI calculated from thestop-time catalyst temperature TCAT_LAST and the like is used as aparameter representative of the temperature state of the three-waycatalyst 5 upon start of the engine 1. Instead, a detection resultprovided by the temperature sensor may be used as such a parameter. Forexample, the start-time catalyst temperature TCAT_INI may be replacedwith a detected temperature value THCM provided by the humidity sensor22 integrated with a temperature sensor, which is disposed downstream ofthe adsorbent 16. Alternatively, as indicated by broken lines in FIG. 1,a catalyst temperature sensor 29 may be attached to the three-waycatalyst 5 to use a detected temperature value TCAT from the catalysttemperature sensor 29. Again, in this strategy, the detected temperaturevalue THCM or TCAT detected upon starting is used only as a parameterindicative of the temperature state of the three-way catalyst 5 uponstarting, in a manner similar to the start-time catalyst temperatureTCAT_INI, thereby providing similar advantages to those of theaforementioned embodiment.

FIG. 11 illustrates in block diagram form an exhaust gas purifyingapparatus according to a second embodiment of the present invention. Theillustrated exhaust gas purifying apparatus differs from the firstembodiment only in the positions of the three-way catalysts andswitching valve. Components identical to those in the first embodimentor components having equivalent functions to those in the firstembodiment are designated the same reference numerals, and detaileddescription thereon is omitted. As illustrated in FIG. 11, the exhaustgas purifying apparatus comprises a pair of upstream and downstreamthree-way catalysts 5A, 5B, each of which contains a three-way catalyst(not shown), within the exhaust pipe 4 of the exhaust system 2 in theengine 1. A passage between the three-way catalysts 5A, 5B in theexhaust pipe 4 is branched into a main passage 13 and a bypass passage14 which circumvents the main passage 13. An absorbent 16 is filled inthe bypass passage 14. A switching valve 35 is disposed in the mainpassage 13, and is controlled by the ECU 25 to open and close throughthe switching valve driver 19. The rest of configuration is similar tothe first embodiment, including the routine executed by the ECU 25 tocontrol opening/closing of the switching valve 35.

With the foregoing configuration, in the second embodiment, theswitching valve 35 is switched to a close position to fully close themain passage 13 when the adsorption control execution condition is metupon start of the engine 1. In this state, exhaust gases passing throughthe upstream three-way catalyst 5A are entirely passed to the bypasspassage 14, and flow into the downstream three-way catalyst 5B afterhydrocarbons within the exhaust gases are adsorbed by the adsorbent 16,thereby preventing hydrocarbons from being emitted to the atmosphere.Subsequently, the CPU switches the switching valve 35 to the openposition to fully open the main passage 13 when an accumulated exhaustgas calory value SGM_Q calculated in a manner similar to the firstembodiment exceeds a determination value TMTRSTIM indicative of thetemperature at which the upstream three-way catalyst 5A is activated andset in accordance with the atmospheric pressure PA, based on thedetermination that the upstream three-way catalyst 5A is activated andthe adsorbent 16 has been heated to a temperature at which hydrocarbonscan be desorbed therefrom. In this state, exhaust gases are purified bythe activated upstream three-way catalyst 5A through itsoxidation/reduction catalyst actions. Also, part of exhaust gases flowstoward the adsorbent 16, and hydrocarbons desorbed from the adsorbent 16is sent to the downstream three-way catalyst 5B together with theexhaust gases and purified thereby.

Further, the CPU may switch the switching valve 35 to the open positionto fully open the main passage 13 when the accumulated exhaust gascalory value SGM_Q calculated in a manner similar to the firstembodiment exceeds a predetermined determination value TMTRSTIMindicative of a temperature at which the downstream three-way catalyst5B is activated, and set in accordance with the atmospheric pressure PA,based on the determination that the downstream three-way catalyst 5B isactivated, and the adsorbent 16 has been heated to the temperature atwhich hydrocarbons can be desorbed therefrom. In this sate, a majorityof exhaust gases, after passing through the main passage 13, flows intothe activated downstream three-way catalyst 5B and is purified therebythrough its oxidation/reduction catalyst action. Further, in this state,the switching valve 35 is operated to apply a portion of exhaust gasesinto the bypass passage 14 to slowly heat the adsorbent 16 with the heatof the applied exhaust gases, thereby promoting the desorption ofhydrocarbons. In the foregoing manner, the second embodiment is similarto the first embodiment in that the switching valve 35 can be switchedto the main passage 13 at an optimal timing immediately after theupstream or downstream three-way catalyst 5A, 5B is activated, inaccordance with an actual activated state of the upstream or downstreamthree-way catalyst 5A, 5B, thereby achieving an optimal exhaust gascharacteristic.

In the second embodiment, the accumulated exhaust gas calory value SGM_Qmay be corrected likewise in accordance with the atmospheric pressure PAinstead of setting the determination value TMTRSTIM in accordance withthe atmospheric pressure PA. Further, in the second embodiment, thedetected temperature value THCM from the humidity sensor 22 integratedwith a temperature sensor, located downstream of the adsorbent 16 may beused as a parameter indicative of the temperature state of the upstreamor downstream three-way catalyst 5A, 5B upon starting, instead of theestimated start-time catalyst temperature TCAT_INI found through acalculation. Alternatively, catalyst temperature sensors 29A, 29B may bemounted to at least one of the upstream and downstream three-waycatalysts 5A, 5B, as indicated by broken lines in FIG. 10, to use adetected temperature value TCAT thereof.

Further, in the foregoing embodiments, the external air temperature TAis used as a parameter indicative of the ambient temperature around theengine 1, however, the external air temperature TA may be replaced withan intake air temperature detected by an intake air temperature sensordisposed on the intake pipe 1 a. Otherwise, details in the configurationmay be modified as appropriate without departing from the spirit andscope of the present invention.

As described above in detail, the exhaust gas purifying apparatus for aninternal combustion engine according to the present invention can switchthe switching valve at an optimal timing in accordance with an actualactivated state of the catalyzer, thereby achieving an optimal exhaustgas characteristic.

1. An exhaust gas purifying apparatus for an internal combustion enginefor purifying exhaust gases discharged from said internal combustionengine, and temporarily adsorbing hydrocarbons within exhaust gases uponstart of said internal combustion engine, said exhaust gas purifyingapparatus comprising: a catalyzer disposed in an exhaust system of saidinternal combustion engine for purifying exhaust gases; an adsorbentfilled in a second passage in said exhaust system for adsorbinghydrocarbons within exhaust gases, said second passage circumventing afirst passage; a switching valve operable to switch between an openposition for opening said first passage and a closed position forclosing said first passage; atmospheric pressure state detecting meansfor detecting an atmospheric pressure state; start-time temperaturestate detecting means for detecting a temperature state of said exhaustsystem upon start of said internal combustion engine; and post-startexhaust gas calory calculating means for calculating the calory ofexhaust gases discharged after the start of said internal combustionengine, switching valve driving means for driving said switching valveto said closed position upon start of said internal combustion engine,and for driving said switching valve to said open position in accordancewith the detected atmospheric pressure state, the detected start-timetemperature state of said exhaust system, and the calculated post-startexhaust gas calory.
 2. An exhaust gas purifying apparatus according toclaim 1, wherein said start-time temperature state detecting meansincludes: stop-time temperature detecting means for detecting thetemperature of said exhaust system at the preceding stop of saidinternal combustion engine; and inoperative time measuring means formeasuring an inoperative time from the preceding stop to the currentstart of said internal combustion engine, wherein said start-timetemperature state detecting means is configured to find the start-timetemperature state of said exhaust system in accordance with the detectedstop-time temperature of said exhaust system and the measuredinoperative time.
 3. An exhaust gas purifying apparatus according toclaim 2, wherein said start-time temperature state detecting meansfurther includes: an ambient temperature detecting means for detectingthe ambient temperature around said internal combustion engine, whereinsaid start-time temperature state detecting means is configured to findthe start-time temperature state of said exhaust system in accordancefurther with the detected ambient temperature.